A typical integrated circuit may include millions of devices or sub circuits. Some of the integrated circuits contain numerous identical sub circuits. For example, a memory chip may contain millions of identical memory cells, and even if only one memory cell has defects, the entire chip may be rendered defective.
To increase yield, redundant memory cells are manufactured on the same chip. If some of the primary memory cells have defects, redundant memory cells can be used to replace the defective primary memory cells. This redundant configuration permits the semiconductor memory device to continue to operate in a normal state. The primary memory cells and redundant memory cells are all connected via fuses controlled by control circuits on the chip. As stated above, if a defective memory cell is discovered, a fuse coupled to the defective memory cell is blown and the redundant memory cell is connected instead. Accordingly, the semiconductor chip with defective primary memory cells can operate normally. This methodology is also used on other integrated circuits, particularly circuits having multiple identical units.
Highly integrated semiconductor memory devices have a fairly high manufacturing cost, which causes a significant loss if any defective cells are discovered. Types of fuses deployed in such semiconductor memory devices include electrical fuses selectively cut by the flow of excessive current and laser fuses selectively cut by an applied laser beam. In contemporary systems, laser fuses are widely used due to their simplicity in use and layout. Electrical fuses are commonly used in semiconductor memory devices such as electrically erasable programmable read only memory (EEPROM) while the laser fuses are very often used in dynamic random access memory (DRAM).
FIG. 1 illustrates a conventional laser fuse structure. A laser fuse 100 is formed close to the top surface of the chip. Through layers of metal islands 112 and vias 104, the laser fuse 100 is connected to two conductive lines 106, which are in turn connected to integrated circuits. By etching and opening a fuse window 118, a thin passivation layer 122 is left, and laser fuse 100 can be broken by a laser. An open circuit is formed between the conductive lines 106. A redundant circuit will then replace the malfunctioning circuit. The fuse structure is typically protected by a protection ring 116. Typically, multiple fuses are formed, connecting to multiple redundant circuits.
The laser fuse components 100, 112 and 104 are typically formed of conductive material like copper or aluminum. For a period of time after the laser burning and before packaging, copper is exposed. Copper is prone to corrosion from detrimental chemical elements such as fluorine, chlorine, sulfur, and the like. These detrimental chemicals may come from ambient environment, from which integrated circuits are fabricated, or from the wafer container, or even from other processes performed after the fabrication. Corrosion may result in the degradation of the repaired integrated circuits. A scrap rate, which measures a percentage of the wafers that become defective again after repairing, may reach as high as 26 percent in certain circumstances, thus the production yield is significantly reduced.
Therefore, what is needed is a method for eliminating/reducing fuse corrosion and for decreasing the scrap rate.